1. Field of the Invention
The present invention generally relates to optical communication equipment and, more specifically but not exclusively, to optical devices for coupling light into and out of photonic integrated circuits.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Photonic integrated circuits (PICs) are used for various applications in telecommunications, instrumentation, and signal-processing fields. A PIC typically uses optical waveguides to implement and/or interconnect various on-chip components, such as optical switches, couplers, routers, splitters, multiplexers/demultiplexers, modulators, amplifiers, wavelength converters, optical-to-electrical (O/E) and electrical-to-optical (E/O) signal converters, etc. A waveguide in a PIC is usually an on-chip solid light conductor that guides light due to an index-of-refraction contrast between the waveguide's core and cladding.
For proper operation, a PIC typically needs to efficiently couple light between an external optical fiber and one or more of on-chip waveguides. An exemplary grating coupler that can be used for this purpose is disclosed, e.g., in U.S. Pat. No. 7,065,272, which is incorporated herein by reference in its entirety. However, one problem with some grating couplers is that they work well only when implemented using materials that provide a relatively high index-of-refraction contrast between the core and both the upper and lower cladding layers, whereas certain active optical elements of the PIC require the use of materials that can provide only a relatively low index-of-refraction contrast.